Abstract: This study aims at providing systematically insights to clarify the impact of cathodic polarization on the stress corrosion cracking (SCC) behavior of 21Cr2NiMo steel. Slow-strain-rate tensile tests demonstrated that 21Cr2NiMo steel is highly sensitive to hydrogen embrittlement at strong cathodic polarization. The lowest SCC susceptibility occurred at −775 mV vs. SCE, whereas the SCC susceptibility was remarkably higher at potentials below −950 mV vs. SCE. Scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) revealed that the cathodic potential decline caused a transition from transgranular to intergranular mode in the fracture path. The intergranular mode transformed from bainite boundaries separation to prior austenitic grain boundaries separation under stronger cathodic polarization. Furthermore, corrosion pits promoted the nucleation of SCC cracks. In conclusion, with the decrease in the applied potential, the SCC mechanism transformed from the combination of hydrogen embrittlement and anodic dissolution to typical hydrogen embrittlement.